The disclosure relates to gas turbine engine augmentors. More particularly, the disclosure relates to augmentor piloting.
High performance military gas turbine engines are usually equipped with an augmentor (afterburner) for increasing the thrust output of the engine. The augmentor is typically located at the end of the turbomachinery in a duct just upstream/ahead of the exit nozzle of the engine. The augmentor provides additional thrust by injecting additional fuel and burning it in this duct. Combustion of the additional fuel typically consumes the remaining oxygen in the flow.
To maximize the additional thrust: a pilot flame is stabilized in the duct; the additional fuel and residual air are well distributed/mixed; and the chemical reaction is at least nearly completed within the augmentor duct before entering the exhaust nozzle.
When the augmentor is not in use, it creates a pressure loss (known as a “dry” pressure loss). Recent augmentor configurations have been developed to reduce the dry pressure loss. In such augmentors, the flame is stabilized on the rear/trailing/downstream edge/face of the exit vanes with close-coupled fuel injection ports (e.g., U.S. Pat. No. 5,685,140). This helps to avoid pressure loss characteristic of so-called “v-gutter” flameholders and other similar flame stabilization devices. Fuel is injected upstream of the downstream face of the vane to provide a desired fuel-air mixture to the wake region downstream of the flameholder for flame stabilization. For further decreases in the dry pressure loss and/or to provide other tactical advantages, the width of the flame stabilization surface is minimized. Reduction in the vane width reduces the ability of the vane to stabilize the flame.
To increase the robustness of these vane-stabilized flames, systems with continuous pilots have been developed. Rearward facing annular steps along the inside diameter (ID) or outside diameter (OD) of the augmentor have been designed to stabilize the flame (e.g., U.S. Pat. Nos. 4,798,048 and 5,230,214). Exhaust products from this flame naturally draft respectively outward or inward along the rear face of each vane and stabilize the combustion process along the rear of the vanes. For more robust flame stabilization, pilot burner(s) may be utilized with a high velocity jet exhaust directed along the rearward facing edge of the vane (e.g., U.S. Pat. No. 5,385,015). Augmentor configurations exist with the pilot gases traveling either from the OD inward or ID outward.
Problems with such designs include the difficulty to guarantee that the pilot flame gases will be transported along nearly the full radial length/span of the vane. Also, despite continuous pilot operation, the flame is not held robustly to the vane, leading to flame dynamics (rumble and/or screech) or to blowout. Furthermore, it is difficult to simultaneously achieve the desired fueling rate for optimum flame stabilization in the wake of the vane while still obtaining uniform fuel injection/spreading across the flow field to attain high combustion efficiency. Injection of fuel along the side of the vane must satisfy both of these constraints; so control of fuel injection to maintain a combustible mixture along the fuel length of this vane wake region, may produce a poor distribution of fuel and air in the remainder of the duct, leading to augmentor inefficiency. Furthermore, it is even more challenging to develop one design to satisfactorily meet such demands over the entire operating flight envelope or substantial portion thereof.
It is therefore desirable to provide an augmentor with an advantageous combination of: continuous and stable anchoring of the flame across the stabilization surface; independent control of flame stabilization; and overall augmentor efficiency.